Studies of the anatomy of the knee require quantitative characterization of structural parameters. As one particular example, anatomical studies of knees in identified populations require a measure of knee osteoarthritis (OA), a slow progressive disease characterized by loss of cartilage in the joint and leads to loss of joint movement and increased pain. Of the two primary compartments (lateral and medial) of the knee, OA is seen mainly in the medial compartment, due to the higher weight-bearing load borne here. Longitudinal evaluation of the disease in an individual relies on clinical and radiographic features, chiefly pain, disability and structural changes. Disease-modifying therapies are currently under development and these will rely upon the accurate and precise assessment of the progression of the disease.
In current practice, the primary endpoint used in population studies, clinical trials, and epidemiological studies of OA of the knee, is the surrogate measure of radiographic minimum Joint Space Width (mJSW), measured between either of the weight-bearing surfaces of the femoral condyles and the tibial plateau from a radiograph taken in a semi-flexed position, as shown in the radiograph of FIG. 1. The progressive loss of cartilage is measured by narrowing of the mJSW. Since OA is seen mainly in the medial compartment, the mJSW is usually measured in this compartment only.
The measurement of mJSW is usually made by a trained physician using a graduated hand-held lens while reviewing the radiograph on a light-box. Using this method, it is difficult to avoid significant inter- and intra-observer variation due to the subjectivity of the human observer. Moreover, since the mJSW is usually only measured in the medial compartment, it is possible that OA in the lateral compartment will elude detection, and indeed, some patients have primary lateral compartment disease. Finally, in that the mJSW is a single measure of disease progression, effects in the whole joint may be masked by reliance on a single indicator.
In order to address the major problem of human subjectivity, computer analysis of digitized knee radiographs for the measurement of mJSW has been employed by several authors. One method of computer analysis for deriving mJSW from X-ray images was described by Duryea et al., 27 Med. Phys., pp. 580-91 (March, 2000), herein incorporated by reference. Moreover, surrogate outcome measures for characterizing the knee joint space other than the mJSW, have been studied and compared with mJSW, for example, by Duryea et al., 11 Osteoarthritis & Cartilage, pp. 102-110 (2003), which is incorporated herein by reference. The foregoing methods are examples of feature-based analytical techniques.
It is desirable, however, to supplement radiographic mJSW measurement with a technique that contributes to greater sensitivity to OA in quantitative measures than is available using non model-based approaches, particularly with a view to clinical evaluation of potential therapies that may be performed more quickly with fewer patients by virtue of the enhanced sensitivity.